The present invention relates to electric power generators and, more particularly, to a mechanically powered electric power generator.
There is currently an increased interest in electrically powered devices, such as radios and flashlights, which can be energized by manual or mechanical power input. Such devices are desirable in emergency situations or in remote locations where neither batteries nor electricity are readily available. U.K. published Patent Application No. 2,304,208A discloses one such power generator system using mechanical power from a wind-up spring to drive a small electric power generator through a gear train for providing electric power to a radio. In addition, applicants are aware of a commercially available flashlight which incorporates an alternating current (ac) generator having a flywheel that can be spun by a ratchet and gear assembly coupled to a finger operated, reciprocating trigger mechanism.
One problem characteristic of these exemplary mechanically powered electric generators is that the mechanical to electrical power conversion efficiency is limited by the relatively slow motion of the mechanical power input, i.e., the speed at which a person can repetitively squeeze a trigger or how quickly a spring can unwind under load. Power conversion in an electric power generator using ferromagnetic elements is based on Faraday's law which relates the electromotive force to the time rate of change of magnetic induction. The higher the frequency of magnetic field change in a coil, the higher the voltage generated at the coil terminals. The peak-to-peak voltage developed across a rotating coil in a generator is given by V=k.omega.=(PnBA).omega., where k=PnBA is called the torque constant, P is the number of poles, .omega.=2.pi.f is the angular frequency, n is the number of turns in the coil, B is the magnetic induction field being cut by turns of the coil, and A is the area of the coil. For a 1 cm.sup.2 area coil where B=0.1 T, n=100 turns, and P=2 poles, obtaining an output voltage V=10 volts requires a frequency f of 800 Hz. The frequency requirement can be lowered if the number of turns is increased, but this is limited by the increased resistance of the wire which results in ohmic losses.
The frequency requirement is onerous since the frequency of repetitive human motion under load is generally limited to less than 10 Hz, and 1 Hz is more typical if significant torque (higher load) is to be delivered. This limitation makes it difficult to maintain operation of, for example, a trigger driven flashlight for extended periods. Slower repetition can allow the time to be extended by stepping up the effective mechanical frequency to a higher frequency by use of a gear train with a 100 to 1 or 1000 to 1 ratio. The difficulty is that beyond 100:1, there are significant frictional losses in the moving parts of such gear trains. A similar problem exists with spring motors in that the time for the spring to unwind can be extended by use of a gear train but the gear train adds frictional losses. Accordingly, it would be advantageous to have a generator which operated efficiently at a lower frequency.